nologin displays a message that an account is not available and exits
non-zero. It is intended as a replacement shell field for accounts that
have been disabled.
If the file /etc/nologin.txt exists, nologin displays its contents to the
user instead of the default message.

And /sbin/nologin is a static binary. It's source code is attached below, in the event you wish to review it.

Code:

/* $OpenBSD: nologin.c,v 1.5 2003/07/10 00:00:58 david Exp $ */
/*
* Copyright (c) 1997, Jason Downs. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR(S) ``AS IS'' AND ANY EXPRESS
* OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
/* Distinctly different from _PATH_NOLOGIN. */
#define _PATH_NOLOGIN_TXT "/etc/nologin.txt"
#define DEFAULT_MESG "This account is currently not available.\n"
/*ARGSUSED*/
int main(int argc, char *argv[])
{
int nfd;
ssize_t nrd;
char nbuf[BUFSIZ];
nfd = open(_PATH_NOLOGIN_TXT, O_RDONLY);
if (nfd < 0) {
write(STDOUT_FILENO, DEFAULT_MESG, strlen(DEFAULT_MESG));
exit (1);
}
while ((nrd = read(nfd, nbuf, sizeof(nbuf))) != -1 && nrd != 0)
write(STDOUT_FILENO, nbuf, nrd);
close (nfd);
exit (1);
}

Thanks! So when I create these system accounts for samba would you recommend that I create them with the false shell? Currently I do not see that as an option on adduser. Do I have to edit the /etc/passed?